In rotary drilling wells (e.g., oil, gas, geothermal wells and the like), a drilling fluid is typically pumped downwardly through a string of hollow drill pipe to, and outwardly through, a drill bit connected at the bottom end of the drill pipe string. The drilling fluid—commonly referred to as a drilling “mud”—is circulated, by injection-pump pressure, back upwardly from the bottom of the drill pipe string, through the annular space formed between the drill pipe and the surrounding well bore and/or casing, to thereby transport drilled cuttings and other well debris out of the borehole.
The following important functions are served by the drilling fluid, or mud: to clean the bit and bottom of the borehole so that the bit can drill; remove and transport drilled cuttings to the surface; cool and lubricate the bit and drill string; maintain pressure balance between the geological formations and the wellbore thereby preventing the loss of well control (blowouts) and mitigating the caving in of the borehole in weak formation regions; seal porous formations; and provide, with routine testing of the mud, information about the formation penetrated. Various materials are added to prepare and condition drilling muds.
The term “mud” applied to drilling fluids contrasts with the fact that drilling fluids are typically quite complex, specially-formulated fluids which play an important, if not critical, role in rotary well drilling operations, especially those in which deep oil or gas wells are being drilled. Drilling muds are, as an illustration, usually formulated to have: (i) a comparatively low viscosity at all temperatures normally encountered, from the surface to the bottom of the borehole, so as to minimize lost circulation problems and costs; (ii) a sufficient density and rheological characteristic to keep drilled cuttings in suspension, to thereby prevent settling of the entrained cuttings in the borehole; and (iii) a relatively high flash point so as to reduce the risk of fires and explosions which might otherwise be caused by use of the drilling mud.
Typically, the base fluids selected for drilling muds are water or oil. Oil base drilling fluids provide many advantages over water base drilling fluids because of their excellent shale inhibition, high penetration rate, temperature stability, and lubricating characteristics. They are typically based on mineral oils containing aromatics, diesel oil, distillates or synthetic oils with the addition of additives therein to effect an emulsion in which the oil provides a continuous phase, e.g., a water in oil invert emulsion drilling fluid. However, the human health risks and environmental pollution caused by the use of diesel, distillates and some mineral oils have become increasingly objectionable because the oils tend to come into contact with workers and escape into the environment, contaminating both the land and the water. This has led to the imposition of strict regulations for the protection of workers and the environment. Spent drilling fluids and drilled cuttings are thus among the most significant waste streams from oil and gas exploration and production operations, and pose a serious and costly disposal problem for onshore and offshore operators. For example, offshore operators must barge spent mud and cuttings to shore for land disposal if the materials do not meet EPA discharge limitations or permit requirements. Due to problems of toxicity and persistence which are associated with these oils, considerable effort and expense has been made by the industry to develop alternatives to such aromatic-containing oils in drilling muds. More recently, highly refined relatively low aromatic mineral oils have been used as the oil phase of drilling fluids. These are of much lower toxicity, but are much more expensive than diesel fuel and distillates and are produced from petroleum feedstocks by a variety of processes which reduce the concentration of aromatics and especially polynuclear aromatics.
There exists a need for less expensive non-aqueous drilling fluid alternatives, particularly those that decrease the health and safety risks posed to employees and the environment and that retain or improve on the rheological characteristics of the more conventional oil base drilling fluids, e.g., diesel oil.